Sky and Telescope - June 2017 - 39

The proper way to install an eyepiece solar filter
- that is, never! See article for why not.

scope about f/13, which would make
the image a little dim but still useable.
If you're making ﬁlters for binoculars
or a 60-mm or smaller scope, you can
make them full aperture.
I get my ﬁlter material from Scope
stuff.com. They sell it in 10″ × 10″ or
8.2″ × 11.7″ sheets, so get together with
some friends and buy a sheet that will
make ﬁlters for all of you.
Get some ﬂat cardboard (not corrugated cardboard); cereal boxes or poster
board work great for this. Cut some long
strips about 2″ wide and wrap them
around the front of your scope, making
a spiral about four or ﬁve layers thick.
Glue the layers of this spiral together
(but not to the scope!) The spiral should
ﬁt snugly but still be possible to slide

on and off. Make sure the layer edges at
the top of the spiral are nice and even:
Those combined edges are going to be
the glue surface for your next step.
While the spiral's glue is drying, cut
out two big circles of cardboard that
will span the entire outside diameter of
the spiral. Then cut your ﬁlter's aperture hole in the disks. For refractors
these holes should be centered, but for
reﬂectors you want to offset them so
your secondary mirror isn't in the light
path (or at least not in the center of the
light path). Leave half an inch or so of
cardboard on the outer edge, partly for
strength and partly because your primary mirror doesn't extend that far out
toward the edge of the tube anyway.
Now choose a side of one of the disks
to be the front and paint that white.
This will help keep the ﬁlter cell cool
under direct sunlight. Also paint the
outside of the spiral ring white. Paint
one side of the other disk black. You
can paint the inside of the spiral black
or just leave it cardboard. I leave mine
cardboard so it doesn't shed paint ﬂakes
down onto my primary mirror.
After the paint dries, align the
cardboard disks so the unpainted sides
are together and the aperture holes are
aligned. Mark the edges of the disks so
you can sandwich them in exactly this
same orientation later.

You can make a camera lens filter in a similar fashion as described in the text by sandwiching
Baader Astrosolar film between two plastic disks and securing them inside a lens hood.

Top: Make a spiral edge out of strips of thin
cardboard. Cereal boxes work well for this.
Below: Cut two disks from your cardboard and
sandwich the filter material between them.

Put a ring of double-stick tape
around the aperture hole on the
unpainted side of one of the big disks.
You'll have to use little pieces to go
around the circle. Lay the ﬁlter material on a piece of soft tissue paper,
then lower the cardboard ring with its
exposed double-stick tape gently onto
the ﬁlter material, being careful to leave
half an inch of ﬁlter material overlapping on all sides of the aperture hole.
If all goes well, the ﬁlter material will
be held ﬁrmly in place and will have no
wrinkles. Some gentle wrinkling is okay,
but if you have serious creases in it,
carefully loosen it and try again.
When it's bonded smoothly to this
ﬁrst disk, put more double-stick tape
on the unpainted face of the other disk,
not just around the hole but in strategic
spots all over, and then spread some
glue around, too, for good measure.
Line up the marks you made earlier and
press the two circles together, making a sandwich with the ﬁlter material
s k y a n d t e l e s c o p e .c o m

* JUNE 2017

http://www.Scopestuff.com http://www.Scopestuff.com http://www.skyandtelescope.com

Sky and Telescope - June 2017

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